It is known that a functional thin film can be formed by coating or printing a film-forming paste on a substrate followed by calcination. This film forming process is advantageous in that no special equipment is required and that a thin film of relatively large area can be prepared with ease. A resistive film of a thermal head, a transparent conductive film, etc. have been produced by this process. For example, a resistive film of a thermal head can be formed by applying a paste composition prepared by dispersing a powder mixture of ruthenium oxide and glass frit in an organic vehicle consisting of a solvent and a resin to a substrate by screen printing followed by calcination as disclosed, e.g., in JP-A-53-100496 and JP-A-54-119695 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). The paste composition used here must be heated to a high temperature around 800.degree. C. in order to fuse glass frit, etc. and to improve adhesion to a substrate.
A paste composition comprising an metal-containing organic compound capable of becoming a transparent conductive metal oxide on calcination, an organic solvent, a resin, and a subliming pigment has been proposed as a paste with improved screen printability for forming a transparent conductive film as disclosed in JP-A-59-18769.
Thus, the conventional functional films have been formed by using a paste composition containing specific additives designed for the particular use. That is, a general-purpose paste composition which is broadly applicable to various types of functional films is unknown.
A paste composition for forming a functional film is required (i) to exhibit satisfactory adhesion to a substrate, (ii) to be applied by any printing method, (iii) to provide a film having uniformity in thickness and composition and no defects such as cracks and pinholes, and (iv) to be able of being calcined at a relatively low temperature so as to broaden the choice of material for a substrate. It is also required to undergo no change of itself with time.
For the time being, a paste composition satisfying all of these requirements is unknown, and it has been keenly demanded to develop an improved paste composition.
On the other hand, the optimum viscosity of a film-forming paste composition is subject to variation according to a desired film thickness or a printing or coating method adopted. Conventionally employed resins for viscosity adjustment include cellulose resins, e.g., ethyl cellulose and nitrocellulose, and butyral resins. However, since ethyl cellulose and a butyral resin not only have a high thermal decomposition temperature but hardly decompose completely, a paste composition containing these resins needs a high calcining temperature. This narrows the choice of substrate material on which a functional film can be formed. Further, a transparent conductive film prepared from a paste composition containing these resins suffers from reductions in performance properties, such as conductivity. To the contrary, nitrocellulose has a low thermal decomposition temperature and completely decomposes at a low temperature. However, nitrocellulose is difficult to handle due to its own explosiveness and is also less available.